There is always a need for precise and reliable metrology to monitor the properties of thin films, especially in the semiconductor and magnetic head industries. Thin film properties of interest include the thickness of one or more layers, the surface roughness, the interface roughness between different layers, the optical properties of the different layers, the compositional properties of the different layers and the compositional uniformity of the film stack. Ellipsometers are particularly well suited to this task when the thickness is less than 100 nm, when there are more than two layers present or when there are compositional variations. Additionally, dimensional measurements such as linewidth, sidewall angle and height can be extracted using ellipsometry.
An ellipsometer is a measurement tool used to determine the change in polarization state of an electromagnetic wave after interaction with a sample. The determination of this polarization state can yield information about the thin film properties such as those listed above. In general, an ellipsometer is a polarization-state-in, polarization-state-out device. FIG. 1 shows a simple block diagram of a typical ellipsometer 10, which includes a Polarization State Generator (PSG) 12 that generates an electromagnetic wave of a known polarization state and a Polarization State Detector (PSD) 16 that determines the polarization state of the electromagnetic wave after interaction with a sample 14. In FIG. 1 the interaction is shown in reflection mode, but it should be understood that the interaction may be in transmission mode, i.e., the PSD determines the polarization state of the electromagnetic wave after transmission through a sample.
Different kinds of PSG/PSD configurations have been proposed and developed for ellipsometers. The advantages of each configuration are specific to the kind of extracted information that is desired. In the thin film metrology field, the most popular ellipsometry configurations include a rotating polarizing element. In these systems, the PSG and/or the PSD contain a rotating polarizing element utilizing a polarizer or compensator.
Unfortunately, rotating element configurations require moving parts employing motors, and therefore are more difficult to design into a compact tool. Compactness is a necessity for an application where the metrology module is integrated into a semiconductor process tool. Furthermore, moving components require maintenance and calibration and may degrade the reliability of the metrology tool.
Another kind of ellipsometer that has been extensively developed and used for thin film metrology is the photoelastic modulator ellipsometer (PME). This instrument employs a photoelastic modulator (PM) to change the polarization state of the light as a function of time either before or after reflection from the sample surface. This modulation can also be accomplished using a Pockels cell or liquid crystal variable retarders instead of a PM. One advantage of the PME is the lack of moving parts as the polarization is manipulated electrically.
FIG. 2 is a block diagram of a conventional PME 20. The PSG portion 21 of the PME 20 includes a light source 22 and a linear polarizer 24. The light source 22 generates a collimated beam (monochromatic or broadband radiation) that is transmitted through the linear polarizer 24. The linearly polarized beam is reflected from the sample surface 26 thereby modifying the polarization state of the electromagnetic beam. The PSD portion 27 of the PME 20 includes a PM (or Pockels cell) 28, another linear polarizer 30, and a detector 32. The PM (or Pockels cell) 28 introduces a time-dependent phase between the x- and y-electric field components of the reflected beam in relation to the optical axis of the PM 28. The linear polarizer 30 modulates the intensity of the incoming beam as a function of the phase imposed by the PM, which is a function of time. The detector 32 records the time-dependent intensity of the electromagnetic beam. The detector 32 can be a single element detector for a single wavelength system or a multichannel spectrograph when multiple wavelengths are used. Other configurations of a PME include a single PM in the PSG instead of the PSD, or a PM in both the PSG and the PSD.
Unfortunately, photoelastic modulators and Pockels cells are relatively large and expensive. Consequently, a disadvantage of an ellipsometer configuration employing modulated polarization such as shown in FIG. 2, is the larger size and greater cost relative to an ellipsometer that does not employ modulated polarization.
U.S. patent application Ser. No. 09/929,625, filed Aug. 13, 2001, entitled “Metrology Device and Method Using a Spatial Variable Phase Retarder”, which is incorporated herein by reference described a metrology configuration that advantageously does not use moving parts or a phase modulator to measure a sample. Calibration of the system, however, requires a periodic reference measurement to minimize the effects of intensity variation due to non-uniformity of beam in phase modulation direction and illumination source, thermal and mechanical drift, which can be time consuming. Moreover, optical components of the system, e.g., the variable retarder, are moved out of the beam path during the reference measurement. Thus, there is a need for an improved system in which calibration reference data that can be easily and quickly measured.